The present invention relates generally to ladar systems and, more particularly, to a self-referencing microdoppler ladar receiver and an associated detection method.
Lidar, laser ladar, optical ladar, and ladar (hereinafter collectively termed xe2x80x9cladarxe2x80x9d) are all names for xe2x80x9cladarxe2x80x9d systems utilizing electromagnetic radiation at optical frequencies. The radiation used by ladar is at wavelengths which are 10,000 to 100,000 times shorter than that used by conventional radar. Nonetheless, radiation in the form of photons is scattered by the target and is collected and processed to yield information about the target and/or the path to the target.
Ladar uses the same principle as radar, i.e., the ladar system transmits optical signals to a target, the transmitted optical signals interact with the target, and some of the optical signals are reflected or scattered back to the ladar system where the backscattered signals can be analyzed. The change in the properties of the backscattered signals enables some property of the target to be determined. For example, the round trip time required for the optical signals to travel to the target and back to the ladar system is commonly used to determine the range to the target.
One type of ladar system is a Doppler ladar system that is used to measure the velocity of a target. When the optical signals transmitted from the ladar system strike a target moving towards or away from the ladar system, the wavelength of the light reflected or scattered off the target will be changed slightly. This change is known as a Doppler shiftxe2x80x94hence the term Doppler ladar. If the target is moving away from the ladar system, the return light will have a longer wavelength (sometimes referred to as a red shift) while the target is moving towards the ladar system, the return light will have a shorter wavelength (producing so-called blue shift).
As described by U.S. Pat. Nos. 5,847,816; 5,847,817 and 5,867,257, the contents of each of which are incorporated herein in their entirety, a microdoppler ladar system can be utilized to detect and to obtain the vibration signature of a number of targets. For example, a microdoppler ladar system can obtain the vibration signature of various military targets for target classification, damage assessment, intelligence gathering and the like. By way of further example, a microdoppler ladar system can be utilized to measure the vibrational spectrum of bridges, buildings, pipelines, pumps, aircraft, volcanoes and the like. Accordingly, a microdoppler ladar system can assist in determining the mechanical status of machinery for a variety of purposes. Moreover, a microdoppler ladar system may be able to monitor the vital signs of a remotely located person, such as a witness during a deposition or a lie detector examination.
A conventional microdoppler ladar system includes a transmitter and a coherent receiver. The transmitter includes a master oscillator and an associated power amplifier for generating a primary laser beam that illuminates the target. The coherent receiver is responsive to backscattered signals produced by the interaction of the transmitted laser beam and the target. The coherent receiver can include a phase locked loop for receiving both the backscattered signals and the primary laser beam generated by the transmitter. By phase locking the backscattered signals and the primary laser beam generated by the transmitter, the phase locked loop can generate signals indicative of the range, the velocity and a characteristic signature of the target. Therefore, a conventional microdoppler ladar system requires that the coherent receiver not only detect the backscattered signals, but also be provided with a sample of the primary laser beam generated by the transmitter for purposes of phase locking with the backscattered signals.
It is oftentimes advantageous for microdoppler ladar systems to detect targets at relatively long ranges. However, the range of conventional microdoppler ladar systems is primarily limited by two factors. First, the transmitter must provide a primary laser beam that has sufficient power to obtain useful backscattered signals. Secondly, the master oscillator of the transmitter must be selected such that the coherence length of the master oscillator is somewhat longer than the cumulative distance from the transmitter to the target and then to the receiver in order for the coherent receiver to properly combine the backscattered signals and the primary laser beam. As such, for a conventional microdoppler ladar system in which the transmitter and receiver are colocated, the microdoppler ladar system cannot reliably detect target vibrations if the target is spaced from the master oscillator by a distance that is more than one-half of the coherence length of the master oscillator.
The coherence length lc of a master oscillator is related to the frequency linewidth xcex94xcexd of the master oscillator as follows: lc=xcfx80(c/xcex94xcexd) wherein c is the speed of light. In order to have the long coherence lengths required to detect remote targets, the master oscillator must therefore be designed to have an extremely narrow linewidth. For example, microdoppler ladar systems onboard spacecraft that are designed to detect targets on the earth would be required to have a master oscillator with an extremely narrow linewidth. Likewise, ground-based microdoppler ladar systems designed to detect targets disposed in space would also be required to have a master oscillator with an extremely narrow linewidth. Similarly, the detection and classification of long range airborne targets would also require a master oscillator having an extremely narrow linewidth since the range of the microdoppler ladar system would have to be in excess of 500 km in some situations. Unfortunately, master oscillators, such as fiber optic sources, semiconductor lasers and diode pumped solid state lasers, having linewidths that are sufficiently narrow for these long range applications are not readily available and, even if available, would greatly increase the cost of the resulting microdoppler ladar system.
A self-referencing microdoppler ladar receiver and an associated detection method are therefor provided for detecting the vibration of a target based upon an analysis of backscattered signals without a local reference derived from the transmitter. As such, the self-referencing microdoppler ladar receiver and the associated detection method can detect targets at long ranges since the analysis of the backscattered signals from the target does not require a comparison or phase locking to the primary laser beam emitted by the transmitter. The self-referencing microdoppler ladar receiver and associated detection method is particularly useful for space-to-earth, earth-to-space and long range air-to-air, ground-to-air and air-to-ground applications.
The self-referencing microdoppler ladar receiver includes a frequency shifter for receiving a backscattered signal from the target and for controllably shifting the frequency of the backscattered signal. The frequency shifter can include, for example, an acoustooptic frequency shifter for shifting the frequency spectrum of the backscattered signals. The self-referencing microdoppler ladar receiver also includes an interferometer, such as a Mach Zender interferometer, for directing portions of the frequency shifted signals along first and second paths of unequal length and for combining the portions of the frequency shifted signals to produce first and second output signals. According to one embodiment, the first path of the interferometer includes a delay loop for delaying the respective portion of the frequency shifted signals by a predetermined time relative to the other portion of the frequency shifted signals. In addition, the interferometer can include a coupler for combining the portions of the frequency shifted signals following their propagation along the first and second paths to produce the first and second output signals.
The self-referencing microdoppler ladar receiver also includes a signal processor, such as a voltage controlled oscillator, for providing a feedback signal to control the frequency shifter based upon differences in the respective power levels of the first and second output signals. In particular, the signal processor provides the feedback signal to control the frequency shifter in order to drive the interferometer towards quadrature. Since the feedback signal provided by the signal processor is proportional to the instantaneous frequency of the backscattered signal which, in turn, includes frequency contributions due to the vibrational velocity of the target, the vibration of the target is detectable without locally referencing the primary laser beam generated by the transmitter.
In one embodiment, the self-referencing microdoppler ladar receiver can also include a balanced coherent receiver disposed between the interferometer and the signal processor. The balanced coherent receiver of this embodiment includes first and second detectors for detecting the first and second output signals, respectively. In addition, the balanced coherent receiver can include a differential amplifier for combining the outputs of the first and second detectors to thereby amplify the differences in the respective power levels of the first and second output signals.
According to one aspect of the present invention, the self-referencing microdoppler ladar receiver is one component of a microdoppler ladar system. According to this aspect of the present invention, the microdoppler ladar system also includes a transmitter that, in turn, may include a master oscillator for producing signals to illuminate the target. Since the self-referencing microdoppler ladar receiver does not phase lock or otherwise compare the backscattered signals to the signals generated by the transmitter, the transmitter need not have as narrow of a linewidth and correspondingly need not have as long of a coherence length as the transmitters of conventional microdoppler ladar systems. For example, the transmitter can be selected such that the signals produced thereby have a coherence length that is less than the cumulative distance from the transmitter to the target and from the target to the receiver.
Accordingly to another aspect of the present invention, a method of detecting the vibration of a target without a local reference derived from the transmitter that illuminates the target is provided. The method of this aspect of the present invention controllably shifts the frequency of a signal backscattered from the target and then directs portions of the frequency shifted signals along first and second paths defined by an interferometer. As described above, the first and second paths are of unequal length such that the portion of the frequency shifted signal propagating along the first path is delayed relative to the portion of the frequency shifted signal propagating along the second path. The respective portions of the frequency shifted signal are then combined to produce first and second output signals. Based upon the differences in the respective power levels of the first and second output signals, a feedback signal is provided to control the frequency shift imparted upon the backscattered signal, thereby driving the interferometer towards quadrature. According to one embodiment of this detection method, the first and second output signals are separately detected prior to providing the feedback signal. In addition, the differences between the first and second output signals can be amplified prior to providing the feedback signal based thereupon.
Since the feedback signal is proportional to the instantaneous frequency of the backscattered signal which, in turn, includes frequency contributions due to the vibrational velocity of the target, the vibration of the target is detectable by a receiver without a local reference derived from the transmitter. As such, the self-referencing microdoppler ladar receiver and the associated detection method can operate independently of the transmitter, thereby permitting the vibrational velocity of the target to be reliably determined even through the cumulative distance from the transmitter to a backscattered signal from remote target and from the target to the receiver is greater than the coherence length of the transmitter. The self-referencing microdoppler ladar receiver and the associated detection method are therefore particularly well suited for the detection and analysis of long range targets, such as the detection of earth-based targets from a spacecraft, the detection of targets in space by a earth-based ladar system and the detection of distant aircraft or other airborne objects. Moreover, the self-referencing microdoppler ladar receiver can advantageously be located in many different positions relative to a transmitter and need not be co-located therewith, thereby increasing the flexibility of the microdoppler ladar system and permitting the self-referencing receiver to detect the vibration of different portions of the target.